1. Field of the Invention
The present invention relates to: a semiconductor substrate, which can be used for a high speed MOSFET or the like; a field effect transistor; a method of forming a SiGe layer suitable for the formation of a strained Si layer or the like and a method of forming a strained Si layer using the same; and a method of manufacturing a field effect transistor.
2. Description of Related Art
In recent years, high speed MOSFET, MODFET and HEMT devices have been proposed in which a strained Si layer, which has been grown epitaxially on a Si (silicon) wafer with a SiGe (silicon germanium) layer disposed therebetween, is used for the channel area. In this type of strained Si-FET, a biaxial tensile strain occurs in the Si layer due to the SiGe which has a larger lattice constant than Si, and as a result, the Si band structure alters, the degeneracy is lifted, and the carrier mobility increases. Consequently, using this strained Si layer for a channel area typically enables a 1.5 to 8 fold speed increase. Furthermore in terms of processes, a typical Si substrate produced by CZ methods can be used as the substrate, and a high speed CMOS can be realized using conventional CMOS processes.
However, in order to achieve epitaxial growth of the aforementioned strained Si layer for use as the FET channel area, a good quality SiGe layer must first be grown by epitaxial growth on the Si substrate. Unfortunately, the difference between the lattice constants for Si and SiGe results in crystallinity problems due to dislocation and the like. As a result, the following conventional countermeasures have been proposed.
These countermeasures include a method utilizing a buffer layer in which the Ge composition ratio within the SiGe is altered with a constant gentle gradient, a method utilizing a buffer layer in which the Ge (germanium) composition ratio is altered in a series of steps, a method utilizing a buffer layer in which the Ge composition ratio is altered to a supper lattice state, and a method utilizing a Si off-cut wafer and a buffer layer in which the Ge composition ratio is altered with a constant gentle gradient (U.S. Pat. Nos. 5,442,205, 5,221,413, PCT WO98/00857, and Japanese Unexamined Patent Application, First Publication No. Hei 6-252046).
However, the conventional techniques described above still suffer from the following problem.
Namely, the crystallinity of the SiGe film grown using the above conventional techniques is sufficiently poor that the threading dislocation density does not reach the level required for a device. Furthermore, it is also difficult to achieve a device of low dislocation density which also displays good surface roughness, a factor which can determine whether a device passes or fails when an actual device is manufactured. This surface roughness refers to the effect observed on the surface of irregularities generated by internal dislocation.
For example, in those cases where a buffer layer is used in which the Ge composition ratio varies with a certain gradient, the threading dislocation density can be kept comparatively low, but the surface roughness deteriorates. In contrast, in those cases where a buffer layer is used in which the Ge composition ratio varies across a series of steps, the surface roughness is comparatively low, but the threading dislocation density becomes undesirably large. Furthermore, in the case where an off-cut wafer is used, the dislocation is more likely to exit laterally, and not in the direction of the film growth, although once again a sufficiently low degree of dislocation density is unachievable.
The present invention takes the above problems into consideration, with an object of providing a semiconductor substrate, a field effect transistor, a method of forming a SiGe layer and a method of forming a strained Si layer which utilizes this SiGe layer formation method, and a method of manufacturing a field effect transistor, in which the threading dislocation density is low and the surface roughness is minimal
In order to resolve the problems detailed above, the present invention adopts the configuration described below. Namely, a semiconductor substrate of the present invention comprises a Si substrate on which is formed a SiGe buffer layer constructed of a plurality of laminated layers comprising alternating layers of a SiGe gradient composition layer in which the Ge composition ratio gradually increases from the Ge composition ratio of the base material, and a SiGe constant composition layer which is provided on top of the gradient composition layer and in which the Ge composition ratio is the same as that of the upper surface of the gradient composition layer.
Furthermore, a SiGe layer formation method according to the present invention is a method of forming a SiGe layer on top of a Si substrate, wherein a step for epitaxially growing a SiGe gradient composition layer in which the Ge composition ratio gradually increases from the Ge composition ratio of the base material, and a step for epitaxially growing, on top of the SiGe gradient composition layer, a SiGe constant composition layer in which the Ge composition ratio is the same as that of the final Ge composition ratio of the gradient composition layer, are performed repeatedly to form a plurality of layers on top of the Si substrate, and generate a SiGe layer in which the Ge composition ratio varies with a series of steps inclined relative to the direction of the film growth.
As a result of extensive research on SiGe film formation technology, the inventors discovered that dislocation within the crystal displayed the following tendencies.
Namely, it is thought that during film formation of the SiGe layer, the dislocations generated within the film display a tendency to propagate either at an oblique angle relative to the direction of the film formation, or in a lateral direction (in a direction orthogonal with the direction of the film formation: direction  less than 110 greater than ). Furthermore, the dislocations are more likely to propagate in a lateral direction at a layer interface, although at those interfaces where the composition alters suddenly, the dislocations are more likely to propagate in the aforementioned oblique direction and nucleations of dislocation are likely to occur at higher densities.
Consequently, if film formation is carried out using a simple stepped Ge composition ratio, then it is thought that high densities of dislocations will be generated at the interface sections where a sudden variation in composition occurs, and that the dislocations will be likely to propagate at an oblique angle relative to the direction of the film formation, and that there will consequently be considerable danger of threading dislocation occurring. Furthermore, if film formation is carried out using a simple gentle Ge composition ratio gradient, then it is thought that the lack of sections (interfaces and the like) which offer opportunity for the oblique dislocations to exit in a lateral direction means that the dislocations penetrate through to the surface.
In contrast, in a method of forming a SiGe layer according to the present invention, because a step for epitaxially growing a SiGe gradient composition layer in which the Ge composition ratio gradually increases from the Ge composition ratio of the base material (Si in those cases where the base material for the growing process is a Si substrate, or SiGe in those cases where the base material is a constant composition layer), and a step for epitaxially growing a SiGe constant composition layer on top of the gradient composition layer in which the Ge composition ratio is the same as that of the final Ge composition ratio of the gradient composition layer are repeated a plurality of times, and furthermore because a semiconductor substrate of the present invention comprises a SiGe buffer layer constructed of a plurality of laminated layers comprising alternating gradient composition layers and constant composition layers, this buffer layer comprises a plurality of alternating gradient composition layers and constant composition layers with the Ge composition ratio in a series of inclined steps, and this state enables a SiGe layer to be formed for which the dislocation density is low and the surface roughness is limited.
In other words, it becomes easier for the dislocations to propagate in a lateral direction at an interface, and consequently it becomes less likely that threading dislocations will occur. Furthermore, because the composition variation at the interface is small, the nucleation of dislocations at the interface is suppressed, and dislocation will occur equally throughout the gradient composition layer, and consequently deterioration in the surface roughness can also be suppressed.
In a semiconductor substrate of the present invention, the aforementioned SiGe buffer layer should preferably comprise from 4 to 7 pairs of laminated layers, where each pair comprises one of the aforementioned gradient composition layers and one of the aforementioned constant composition layers.
Furthermore, a method of forming a SiGe layer according to the present invention, should preferably comprise from 4 to 7 repetitions of the steps for epitaxial growth of the aforementioned gradient composition layer and the aforementioned constant composition layer. In other words, if one step is considered to comprise the formation of one gradient composition layer and one constant composition layer, then as described below, increasing the number of steps decreases the threading dislocation density, and in those cases where 4 to 7 step repetitions are used to form the gradient composition layers and the constant composition layers, the threading dislocation density can be reduced to less than half the density observed after only one step.
In another semiconductor substrate of the present invention, the aforementioned SiGe buffer layer should preferably comprise 3 or 4 pairs of laminated layers, where one pair comprises one of the aforementioned gradient composition layers and one of the aforementioned constant composition layers.
Furthermore, another method of forming a SiGe layer according to the present invention, should preferably comprise 3 or 4 repetitions of the steps for epitaxial growth of the aforementioned gradient composition layer and the aforementioned constant composition layer. In other words, as described below, there is an optimum number of steps for producing the maximum reduction in the surface roughness, and in those cases where 3 to 4 step repetitions are used to form the gradient composition layers and the constant composition layers, the surface roughness can be reduced to a minimal value.
In yet another semiconductor substrate of the present invention, it is also effective if the aforementioned SiGe buffer layer is formed so that the thickness of the aforementioned gradient composition layer and the constant composition layer are set so as to gradually decrease with increasing distance from the aforementioned Si substrate.
Furthermore, in yet another method of forming a SiGe layer according to the present invention, it is also effective to gradually decrease the thickness of the gradient composition layer and the constant composition layer in the epitaxial growth steps after each step repetition. Namely, dislocation will nucleate more readily with higher Ge composition, and so if film formation is conducted with a single uniform thickness, then the uppermost layer will display greater dislocation density, whereas according to the method of the present invention, by gradually reducing the thickness of the gradient composition layer and the constant composition layer after each step repetition, a more equal distribution of dislocation can be achieved through each of the layers.
A semiconductor substrate of the present invention is a semiconductor substrate with a SiGe layer formed on top of a Si substrate, and in which the SiGe layer is formed using the SiGe layer formation method described above. In other words, because the SiGe layer in the semiconductor substrate is formed using the aforementioned method of forming a SiGe layer, a superior SiGe layer can be obtained in which the dislocation density is low and the surface roughness is minimal, and this semiconductor substrate is then ideal as a substrate for forming a strained Si layer on top of the SiGe layer.
Yet another semiconductor substrate of the present invention comprises a semiconductor substrate of the present invention described above, wherein a strained Si layer is provided either directly on the SiGe buffer layer of the semiconductor substrate, or provided on the SiGe buffer layer with another SiGe layer disposed therebetween.
Furthermore, a method of forming a strained Si layer according to the present invention is a method of forming a strained Si layer on a Si substrate with a SiGe layer disposed therebetween, comprising a step for epitaxially growing a SiGe buffer layer on a Si substrate using the SiGe layer formation method described above, and a step for epitaxially growing a strained Si layer, either directly on top of the SiGe buffer layer, or with another SiGe layer disposed therebetween.
Furthermore, yet another semiconductor substrate of the present invention is a semiconductor substrate in which a strained Si layer is formed on top of a Si substrate, with a SiGe layer disposed therebetween, wherein the strained Si layer is formed using the strained Si layer formation method of the present invention described above.
The aforementioned semiconductor substrate comprises a strained Si layer provided either directly on the SiGe buffer layer of the semiconductor substrate of the aspect of the present invention described above, or provided on the SiGe buffer layer, but with another SiGe layer disposed therebetween, and furthermore in the aforementioned method of forming a strained Si layer, a strained Si layer is epitaxially grown, either directly on top of the SiGe buffer layer generated by epitaxial growth using the aforementioned SiGe layer formation method, or with another SiGe layer disposed therebetween, and furthermore in the aforementioned semiconductor substrate, a strained Si layer is formed using the strained Si layer formation method according to the present invention, and so the Si layer is formed on the superior surface of the SiGe layer, enabling a superior quality strained Si layer to be produced. This semiconductor substrate is then ideal, for example, as a substrate for an integrated circuit using a MOSFET or the like, in which the strained Si layer is used as a channel area.
A field effect transistor of the present invention is a field effect transistor with a channel area formed on a strained Si layer which has been grown epitaxially on top of a SiGe layer, wherein the channel area is formed on the aforementioned strained Si layer of the aforementioned semiconductor substrate according to the present invention.
Furthermore, a method of manufacturing a field effect transistor according to the present invention is a method of manufacturing a field effect transistor with a channel area formed on a strained Si layer which has been grown epitaxially on top of a SiGe layer, wherein the strained Si layer is formed using the aforementioned strained Si layer formation method of the present invention.
Furthermore, a field effect transistor of the present invention is a field effect transistor with a channel area formed on a strained Si layer which has been grown epitaxially on top of a SiGe layer, wherein the strained Si layer is formed using the aforementioned strained Si layer formation method of the present invention.
In the above field effect transistors and the above method of manufacturing a field effect transistor, because the channel area is formed on the strained Si layer of the aforementioned semiconductor substrate according to the present invention, or alternatively, a strained Si layer on which the channel region is to be formed is formed using the aforementioned strained Si layer formation method of the present invention, the superior nature of the strained Si layer enables a high quality field effect transistor to be produced with a good yield.
According to the present invention, the following effects can be achieved.
According to a semiconductor substrate of the present invention, a SiGe buffer layer is provided which is constructed of a plurality of laminated layers comprising alternating gradient composition layers and constant composition layers, and according to a method of forming a SiGe layer of the present invention, a step for epitaxially growing a gradient composition layer and a step for epitaxially growing a constant composition layer are repeated a plurality of times, forming a SiGe layer in which the Ge composition ratio varies with a series of steps inclined relative to the direction of the film growth, and as a result, the occurrence of high densities of dislocations at the interfaces can be suppressed, and moreover dislocations are able to propagate in a lateral direction so as not to penetrate through to the surface.
Consequently, dislocations, which are essential for lattice relaxation, can be generated more equally throughout the substrate, enabling a reduction in surface roughness, and moreover the dislocations are able to propagate in a lateral direction, enabling film formation to be performed with a reduced occurrence of threading dislocation. As a result, a high level of crystallinity with a low threading dislocation density and a minimal surface roughness can be obtained.
According to a semiconductor substrate with a strained Si layer according to the present invention, a strained Si layer is provided either directly on the SiGe buffer layer of the aforementioned semiconductor substrate of the present invention, or alternatively provided on the SiGe buffer layer with another SiGe layer disposed therebetween, and according to a method of forming a strained Si layer of the present invention, a strained Si layer is grown epitaxially, either directly on top of the SiGe buffer layer generated by epitaxial growth using the aforementioned SiGe layer formation method of the present invention, or with another SiGe layer disposed therebetween, and as a result the Si layer can be formed on the superior surface of the SiGe layer, enabling the formation of a high quality strained Si layer.
Furthermore, according to a field effect transistor of the present invention, a channel area is formed on the aforementioned strained Si layer of the semiconductor substrate of the present invention, and according to a method of manufacturing a field effect transistor of the present invention, a strained Si layer on which the channel region is to be formed is formed using the aforementioned strained Si layer formation method of the present invention, and consequently the resulting high quality strained Si layer enables a high quality MOSFET to be produced with a good yield.